The present invention relates to a time resolved spectroscopy method and apparatus for in vivo characterization of tissue.
Continuous wave (CW) tissue oximeters have been widely used to determine in vivo concentration of an optically absorbing pigment (e.g., hemoglobin, oxyhemoglobin) in biological tissue. The CW oximeters measure attenuation of continuous light in the tissue and evaluate the concentration based on the Beer Lambert equation or modified Beer Lambert absorbance equation. The Beer Lambert equation (1) describes the relationship between the concentration of an absorbent constituent (C), the extinction coefficient (.epsilon.), the photon migration pathlength &lt;L&gt;, and the attenuated light intensity (I/I.sub.o). ##EQU1## The CW spectrophotometric techniques cannot determine .epsilon., C, and &lt;L&gt; at the same time. If one could assume that the photon pathlength were constant and uniform throughout all subjects, direct quantification of the constituent concentration (C) using CW oximeters would be possible.
In tissue, the optical migration pathlength varies with the size, structure, and physiology of the internal tissue examined by the CW oximeters. For example, in the brain, the gray and white matter and the structures thereof are different in various individuals. In addition, the photon migration pathlength itself is a function of the relative concentration of absorbing constituents. As a result, the pathlength through an organ with high blood hemoglobin concentration, for example, will be different from the same with a low blood hemoglobin concentration. Furthermore, the pathlength is frequently dependent upon the wavelength of the light since the absorption coefficient of many tissue constituents is wavelength dependent. Thus, where possible, it is advantageous to measure directly the pathlength when quantifying the hemoglobin concentration in tissue.
Frequently, it is advantageous to determine the hemoglobin saturation in vivo. Although the arterial oxygen saturation in a perfused organ can be quantified, it is not possible to estimate the change in the hemoglobin oxygen concentration as it leaves an artery and enters the capillary bed; nor is it possible to determine the intermediate value of oxygen saturation in a particular capillary bed from the venous drainage since no technique has been devised for drawing a blood sample directly from the capillary bed.
In contrast to CW oximeters, time resolved spectroscopy (TRS-pulse) can measure directly the average pathlength of migrating photons as well as other tissue properties such as the absorption and scattering of light in tissue.
As described in the above-cited patent and patent applications, the TRS system irradiates tissue with pulses of light of 10.sup.-10 sec. duration that migrate through a path between an optical input port and an optical detection port. The shape of the input pulse is modified by the scattering and absorption properties of the tissue. The modified light is detected by a photo-multiplier, amplified and stored in a multichannel analyzer. The multichannel analyzer collects only a single photon for each input light pulse. A signal from each detected photon is encoded for time delay and recorded. The pulses are accumulated over a relatively long time interval (on the order of 5 minutes) so that approximately 10.sup.5 counts are collected at the detected pulse maximum. The relatively long counting time is required to obtain reasonable statistics so that a reasonable fit over three or four decades of the logarithmic slope on the detected pulse can be obtained.
For some applications, the relatively long collection time is a disadvantage. Furthermore, the instrumentation of the single photon counting TRS-pulse system is costly when compared with the CW systems. The relative complexity, cost, and size of the TRS-pulse system, of the embodiment illustrated in U.S. Pat. No. 5,119,815, could present some barriers to marketing for certain applications in today's cost-conscious health care industry.
Thus, there is a need for a cost-effective time resolved spectroscopic system that requires a relatively short period of data accumulation for quantitative and qualitative tissue examination.